Methods and Apparatuses for Transmission Power Control

ABSTRACT

Methods and apparatuses that facilitate power and quality control of uplink MIMO transmissions. A method in a NodeB comprises controlling transmission power of multiple pilot signals transmitted by a user equipment by using a single inner power control loop operating on at least one pilot signal of the multiple pilot signals. Some disclosed embodiments also relate to adjustment of a quality target applied by the single inner power control loop and to updating of a power offset for computing the number of bits that the user equipment can transmit on a stream that is associated with a pilot signal that is not power controlled by the single inner power control loop.

RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/378,293, filed 14 Dec. 2011, which is a national stageapplication for international patent application no. PCT/SE2011/051456,filed 30 Nov. 2011, which claims priority to U.S. provisionalapplication No. 61/426,036, filed 22 Dec. 2010. The entire contents ofeach of the aforementioned applications are incorporated herein byreference.

TECHNICAL FIELD

The embodiments described herein relate to transmission power control ina communications system and in particular to transmission power controlof uplink Multiple-Input Multiple-Output, MIMO transmissions.

BACKGROUND

There is a continuous development of new generations of mobilecommunications technologies to cope with increasing requirements ofhigher data rates, improved efficiency and lower costs. High SpeedDownlink Packet Access (HSDPA) and High Speed Uplink Packet Access(HSUPA), together referred to as High Speed Packet Access (HSPA), aremobile communication protocols that were developed to cope with higherdata rates than original Wideband Code Division Multiple Access (WCDMA)protocols were capable of. The 3rd Generation Partnership Project (3GPP)is a standards-developing organization that is continuing its work ofevolving HSPA and creating new standards that allow for even higher datarates and improved functionality.

In a radio access network implementing HSPA, a user equipment (UE) iswirelessly connected to a radio base station (RBS) commonly referred toas a NodeB (NB). A radio base station is a general term for a radionetwork node capable of transmitting radio signals to a user equipment(UE) and receiving signals transmitted by a user equipment (UE).

3GPP has evaluated the potential benefits of uplink transmit (Tx)diversity in the context of HSUPA. With uplink transmit diversity, UEsthat are equipped with two or more transmit antennas are capable ofutilizing all of them for uplink transmissions. This is achieved bymultiplying a UE output signal with a set of complex pre-coding weights,a so-called pre-coding vector with one pre-coding weight for eachphysical transmit antenna. The rationale behind uplink transmitdiversity is to adapt the pre-coding weights so that user and networkperformance is maximized. Depending on UE implementation the antennaweights may be associated with different constraints. Within 3GPP twoclasses of transmit diversity are considered:

-   -   Switched antenna transmit diversity, where the UE at any given        time-instance transmits from one of its antennas only.    -   Beamforming where the UE at a given time-instance can transmit        from more than one antenna simultaneously. By means of        beamforming it is possible to shape an overall antenna beam in        the direction of a target receiver.

While switched antenna transmit diversity is possible for UEimplementations with a single power amplifier (PA), the beam formingsolutions may require one PA for each transmit antenna.

Switched antenna transmit diversity can be seen as a special case ofbeamforming where one of the antenna weights is 1 (i.e., switched on)and the antenna weight of any other antenna of the UE is 0 (i.e.,switched off).

A fundamental idea behind uplink transmit diversity is to exploitvariations in the effective channel to improve user and networkperformance. The term effective channel here incorporates effects oftransmit antenna(s), transmit antenna weights, receiving antenna(s), aswell as the wireless channel between transmitting and receivingantennas. Selection of appropriate antenna weights is crucial in orderto be able to exploit the variations in the effective channelconstructively.

During 2009 and 2010 the 3GPP evaluated the merits of open loop beamforming and open loop antenna switching for uplink transmissions inWCDMA/HSPA. These techniques are based on that UEs equipped withmultiple transmit antennas exploit existing feedback e.g. feedbacktransmitted on the Fractional Dedicated Physical Channel (F-DPCH) or onthe E-DCH HARQ Acknowledgement Indicator Channel (E-HICH) to determine asuitable pre-coding vector in an autonomous fashion. The purpose ofpre-coding the signals is to “maximize” the signal to interference ratio(SIR) at the receiving NodeB. Since the network is unaware of theapplied pre-coding weights the NodeBs will experience a discontinuity inthe measured power whenever a change in pre-coding weights occurs. Asummary of the 3GPP studies on open loop transmit diversity techniquescan be found in 3GPP's technical report TR 25.863, UTRA: Uplink TransmitDiversity for High Speed Packet Access.

Recently there have been proposals for introducing closed loop transmitdiversity for WCDMA/HSPA. Closed loop transmit diversity refers to bothclosed loop beam forming and closed loop antenna switching. At the 3GPPmeeting RAN#50 a work item with the purpose of specifying support forclosed loop transmit diversity was agreed. Contrary to the open looptechniques where the UE decides pre-coding weights autonomously, closedloop techniques are based on that the network, e.g., the serving NodeB,selects the pre-coding vector with which the signal is multiplied. Inorder to signal the necessary feedback information from the network tothe UE, the NodeB can either rely on one of the existing physicalchannels, e.g., F-DPCH, or a new feedback channel could be introduced.

Uplink multiple-input-multiple-output (MIMO) transmission is anotherrelated technique that has been proposed as a candidate for WCDMA/HSPAin 3GPP standard release 11. A study item on uplink MIMO for WCDMA/HSUPAwas started at the 3GPP RAN#50 plenary meeting. For uplink MIMO,different data is transmitted from different virtual antennas inso-called streams, where each virtual antenna corresponds to a differentpre-coding vector. Note that closed loop beam forming can be viewed as aspecial case of uplink MIMO where no data is scheduled on one of twovirtual antennas.

MIMO technology is mainly beneficial in situations where the “compositechannel” is strong and has high rank. The term composite channelincludes the potential effects of transmit antenna(s), PAs, as well asthe radio channel between the transmitting and receiving antennas. Therank of the composite channel depends on the number of uncorrelatedpaths between the transmitter and the receiver. Single-streamtransmissions, i.e., beamforming techniques, are generally preferredover MIMO transmissions in situations where the rank of the compositechannel is low e.g. where there is a limited amount of multi-pathpropagation and cross polarized antennas are not used, and/or the pathgain between the UE and the NodeB is weak. This results from a combinedeffect of that the theoretical gains of MIMO transmissions is marginalat low SIR operating point and that inter-stream interference can beavoided in case of single-stream transmissions.

Currently HSUPA does not allow MIMO transmission since only singlestream transmissions are allowed. Inner loop power control (ILPC) andouter loop power control (OLPC) are used to control the quality of theuplink transmission. More specifically, the ILPC is located in theNodeB(s) of an active set. The ILPC is used to ensure that a DedicatedPhysical Control Channel (DPCCH) pilot quality target Γ_(target) ismaintained. All NodeB(s) in the active set monitor that the receivedpower of the DPCCH pilot fulfills the quality target Γ_(target) andbased on this monitoring these NodeB(s) issue transmit power control(TPC) commands to the UE to raise or lower the transmission power of theDPCCH pilot. Since gain factors for a certain Enhanced Dedicated ChannelTransmission Format Combination (E-TFC) are pre-defined power offsetswith respect to the DPCCH transmit power, the ILPC implicitly controlsthe transmit power of all the physical channels. The OLPC is located inthe radio network controller (RNC) and is used to control the qualitytarget Γ_(target) used by the ILPC. Although not specified in the 3GPPstandard, the OLPC typically increases the quality target Γ_(target) ifa too high transport block error rate (BLER) is observed.

For uplink MIMO transmissions, the UE needs to transport multiple DPCCHpilots in order to estimate the wireless channel. For instance for 2×2uplink MIMO, two DPCCHs need to be transmitted by the UE. Data signalsassociated with different streams and different pilot signals willgenerally experience different radio link quality. An issue for suchsettings then becomes power control to ensure reliability and efficiencyof UL multiple stream transmissions.

SUMMARY

It is an object to provide methods and apparatuses for power control ofuplink MIMO transmissions. This object may be achieved by means ofmethods, and apparatuses according to the independent claims.

A first embodiment provides a method in a NodeB configured forcontrolling transmission power of a user equipment configured for uplinkMIMO transmissions. The method comprises controlling transmission powerof multiple pilot signals transmitted by the user equipment. Thetransmission power of the multiple pilot signals is controlled by usinga single inner power control loop operating on at least one pilot signalof the multiple pilot signals. The single inner power control loopcomprises a step of generating transmit power control commands. Qualityof the at least one pilot signal and a quality target are consideredwhen generating the transmit power control commands. The single innerpower control loop also comprises a step of transmitting, to the userequipment, the transmit power control commands to adjust thetransmission power of the at least one pilot signal so that the qualityof the at least one pilot signal meets the quality target.

A second embodiment provides a method in a user equipment configured foruplink MIMO transmissions. The method comprises transmitting multiplepilot signals. The method also comprises receiving, from a NodeB,transmit power control commands for adjusting the transmission power ofa first pilot signal. The method further comprises adjusting thetransmission power of the first pilot signal in accordance with thetransmit power control commands while adjusting the transmission powerof a second pilot signal so that a fixed power difference is maintainedbetween the first pilot signal and the second pilot signal.

A third embodiment provides a NodeB configured for controllingtransmission power of a user equipment configured for uplink MIMOtransmissions. The NodeB comprises a processor and a transceiverconfigured to control transmission power of multiple pilot signalstransmitted by the user equipment. The processor and transceiver areconfigured to control the transmission power of the multiple pilotsignals by executing a single inner power control loop operating on atleast one pilot signal of the multiple pilot signals. The single innerpower control loop, when executed, comprises generating transmit powercontrol commands. Quality of the at least one pilot signal and a qualitytarget are considered when generating the transmit power controlcommands. The single inner power control loop also comprisestransmission, to the user equipment, of the transmit power controlcommands to adjust the transmission power of the at least one pilotsignal so that the quality of the at least one pilot signal meets thequality target.

A fourth embodiment provides a user equipment configured for uplink MIMOtransmissions. The user equipment comprises a transceiver and aprocessor. The transceiver is configured to transmit multiple pilotsignals and to receive, from a NodeB, transmit power control commandsfor adjusting the transmission power of a first pilot signal. Theprocessor is configured to adjust the transmission power of the firstpilot signal in accordance with the transmit power control commandswhile adjusting the transmission power of the second pilot signal sothat a fixed power difference between the first pilot signal and thesecond pilot signal is maintained.

An advantage of some of the embodiments described herein is thatmultiple streams of uplink MIMO transmissions may be power controlled.Another advantage of some of the embodiments of this disclosure is thatthe network is provided a possibility to control quality of uplink MIMOtransmissions. Yet another advantage of some of the embodiments of thisdisclosure is that power and quality control of different streams ofuplink MIMO transmissions may be achieved without requiring anysignaling from the UE to the NodeB of power offsets between differentpilot signals for the purpose of channel estimation. By using a singleinner loop power control (ILPC) to control quality for all streams andmaintaining a fixed power offset between the pilot signals, there is noneed to signal the power offset between the different pilot signals.

Further advantages and features of embodiments of the present inventionwill become apparent when reading the following detailed description inconjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a communication system inwhich different embodiments of this disclosure may be implemented.

FIG. 2 is a schematic block diagram illustrating an embodiment of acommunication system supporting uplink MIMO.

FIG. 3 is a schematic block diagram illustrating an embodiment of a userequipment architecture which can support uplink MIMO.

FIG. 4 is a schematic block diagram illustrating an alternativeembodiment of a user equipment architecture which can support uplinkMIMO.

FIGS. 5 a and 5 b are flow diagrams illustrating alternative embodimentsof methods of this disclosure which are performed in a NodeB.

FIGS. 6 a, 6 b and 6 c are flow diagrams illustrating furtheralternative embodiments of methods of this disclosure which areperformed in a NodeB.

FIG. 7 is a flow diagram illustrating a further exemplary embodiment ofa method which is performed in a NodeB.

FIG. 8 is a flow diagram illustrating an exemplary embodiment of amethod which is performed in a user equipment.

FIG. 9 is a flow diagram illustrating an exemplary alternativeembodiment of a method which is performed in a user equipment.

FIG. 10 is a schematic block diagram of a NodeB according to anembodiment of this disclosure.

FIG. 11 is a schematic block diagram of a user equipment according to anembodiment of this disclosure.

DETAILED DESCRIPTION

The embodiments of this disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in whichdifferent exemplary embodiments are shown. These exemplary embodimentsare provided so that this disclosure will be thorough and complete andnot for purposes of limitation. In the drawings, like reference signsrefer to like elements.

Embodiments of this disclosure may be implemented in a network such asthat illustrated in FIG. 1. As shown in FIG. 1, an example network 11may include one or more instances of user equipments (UEs) 13 and one ormore base stations 12 capable of communicating with the UEs 13, alongwith any additional elements suitable to support communication betweenUEs or between a UE and another communication device (such as a landlinetelephone). Although the illustrated UEs 13 may represent communicationdevices that include any suitable combination of hardware and/orsoftware, these UEs may, in particular embodiments, represent devicessuch as the example UE illustrated in greater detail by FIG. 11.Similarly, although the illustrated base stations 12 may representnetwork nodes that include any suitable combination of hardware and/orsoftware, these base stations may, in particular embodiments, representdevices such as the example base station 12 illustrated in greaterdetail by FIG. 10.

FIG. 2 is a schematic block diagram illustrating a system in whichdifferent embodiments of this disclosure may be implemented. FIG. 2shows a UE 13 configured to support uplink MIMO transmissions forcommunication with a network node 12, which for instance may be aserving NodeB. The exemplary UE 13 is illustrated with two physicaltransmit antennas 23, 24 and the network node 12 is illustrated with twophysical receive antennas 25, 26. The composite channel between the UE13 and the network node 12 comprises wireless channels h11, h12, h21 andh22 between the different transmit antennas 23, 24 and receive antennas25, 26 as illustrated in FIG. 2.

Using uplink MIMO, different data, such as a first signal s1(t) and asecond signal s2(t) as illustrated in FIG. 2, are transmitted indifferent streams 21, 22. Here parts with dashed border are associatedwith a first stream 21 and parts with dotted borders are associated witha second stream 22. Signals associated with the first stream 21 arepre-coded with pre-coding weights w1 and w2 prior to transmission fromthe different physical antennas 23 and 24. Signals associated with thesecond stream 22 are pre-coded with pre-coding weights w3 and w4 priorto transmission from the different physical antennas 23 and 24.

For multi-antenna transmission techniques it is important that thenetwork, e.g., a serving NodeB, has the ability to acquire knowledgeabout the wireless channels. This is because for a UE 13 configured inuplink MIMO mode, knowledge about the channel characteristics are neededboth to determine the rank of the channel and to determine suitablepre-coding vector(s).

In the following we assume that the UE 13 transmits a primary pilotsignal on a primary dedicated physical control channel (P-DPCCH) and asecondary pilot signal on a secondary dedicated physical control channel(S-DPCCH). The primary dedicated physical control channel and thesecondary dedicated physical control channel may alternatively bereferred to as dedicated physical control channel (DPCCH) and secondarydedicated physical control channel (S-DPCCH). It is further assumed thatthe transmit power associated with the P-DPCCH is P_(P-DPCCH) and thetransmit power associated with the S-DPCCH is P_(S-DPCCH)=δ·P_(P-DPCCH),where δ is a relative power difference between P-DPCCH and S-DPCCH. Wefurthermore let

$\begin{matrix}{H = \begin{bmatrix}{h\; 11} & {h\; 12} \\{h\; 21} & {h\; 22}\end{bmatrix}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

denote the channel matrix of the wireless channel between the UE 13 andthe network node 12. Here h12 denotes the wireless channel between asecond transmit antenna 24 of the UE 13 and a first receive antenna 25of the network node 12. We also let

$\begin{matrix}{\Omega = \begin{bmatrix}\kappa & 0 \\0 & \eta\end{bmatrix}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

be a matrix summarizing inaccuracies of power amplifiers (PAs)associated with the different physical antennas 13, 14. Note that κ is arandom variable that describes the inaccuracy associated with the first(upper) transmit branch, while η is a random variable describing theinaccuracy of the PA associated with the second (lower) transmit branchillustrated in FIG. 2. Finally, we also let

$\begin{matrix}{W = \begin{bmatrix}{w\; 1} & {w\; 3} \\{w\; 2} & {w\; 4}\end{bmatrix}} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$

represent a pre-coding matrix. Here [w1 w2] are the pre-coding weightsapplied to the P-DPCCH and other signals associated with the firststream 21 and [w3 w4] are the pre-coding weights applied to the S-DPCCHand other signals associated with the second stream 22. With thesenotations, a received signal r=[r₁ r₂] at the NodeB 12 can be written as

r=H·Ω·W·diag([1δ])·s  (Eq. 4)

where s=[s1(t) s2(t)]^(T) are two pilot signals. W corresponds to theidentity matrix in a case where the DPCCH pilots are not pre-coded, asillustrated in FIG. 4 described below.

FIGS. 3 and 4 illustrate two possible architectures for a UE configuredin uplink MIMO mode comprising two physical antennas 23 and 24 andconfigured to use two streams for uplink MIMO transmissions. For uplinkMIMO transmissions, a UE can transmit two of certain physical channels;one for each stream. For demodulation and channel sounding purposes atleast one DPCCH pilot needs to be transmitted for each stream. In FIG. 3the primary DPCCH (P-DPCCH) pilot and the secondary DPCCH (S-DPCCH)pilot are pre-coded with the same pre-coding vectors as used forpre-coding the other physical channels transmitted for each stream. TheP-DPCCH is associated with a stream 31 and the S-DPCCH is associatedwith a stream 32. In FIG. 4 the P-DPCCH and S-DPCCH are not pre-codedand are associated with streams 41 and 42 respectively.

As mentioned above, an issue in case of uplink MIMO is how to ensurethat the radio link quality associated with all virtual antennas can becontrolled. One solution would be to introduce additional ILPCs andOLPCs so that each virtual antenna (i.e. stream) has its individual ILPCand OLPC. However, this solution presents several drawbacks andproblems. For instance the signaling load on the serving RNC (S-RNC)will increase due to that the S-RNC needs control two or more OLPCs.Another drawback is that additional Fractional Dedicated PhysicalChannel (F-DPCH) resources need to be allocated to UEs configured inMIMO mode since each ILPC will require F-DPCH resources. Yet anotherdrawback is that channel estimation for the purpose of channel soundingwill become increasingly difficult. The latter is because the NodeB(s)need to be aware of the relative power difference δ between the DPCCHpilots in order to estimate the channel as can be seen from equation Eq.4 above. The channel estimation is in turn necessary for performing thechannel sounding in which suitable pre-coding vectors and the number ofstreams that should be scheduled is determined.

Apart from increasing downlink overhead, an architecture relying onmultiple ILPC and OLPC loops thus require that the relative powerdifference between the DPCCHs are signaled by the UE. In soft handoverthe UE will receive TPC commands from both the serving and non-servingNodeB. Hence it is not possible for a single NodeB to keep track of thepower difference between the P-DPCCH and the S-DPCCH by monitoring theTPC that it transmitted to the UE in soft handover. Furthermore sinceonly the F-DPCH from the serving NodeB is power controlled by the UE,the non-serving NodeB cannot accurately estimate the pilot powerdifference. In addition, due to the large dynamic that can be expectedin terms of DPCCH transmission power, in case similar SIR targets areused for all streams, several bits would have to be allocated on oneexisting uplink control channel to signal this relative powerdifference. To avoid such redesigns a solution relying on one ILPC andone OLPC may be desirable.

To ensure that the serving and any non-serving NodeB are aware of thepower difference δ, it can be either signaled by the UE 13 or keptconstant. The latter could be achieved with a single ILPC that adjustthe transmit power of both the P-DPCCH and the S-DPCCH.

This disclosure describes methods which allow the network to control thequality of data transmissions when the number of streams—also referredto as layers—transmitted by the UE exceeds the number of inner powercontrol loops. In the following we will focus on a context where thereonly exists a single ILPC that controls the transmit power of both theP-DPCCH and the S-DPCCH. Different embodiments which are based on asingle ILPC will now be described. Some of the described embodimentsrelate to the NodeB and some embodiments relate to the UE.

Some embodiments relate to a method in a NodeB configured forcontrolling transmission power of a user equipment supporting uplinkMIMO transmissions. Consequently the user equipment transmits multiplepilot signals as described above. The NodeB controls transmission powerof the pilot signals by using a single ILPC operating on one or severalof the pilot signals. The ILPC involves generating transmit powercontrol (TPC) commands. Quality of one or several of the pilot signalsand a quality target are considered when generating the TPC commands.The Node B transmits the TPC commands to the user equipment to adjustthe transmission power of the pilot signal(s) on which the ILPC operatesso that the quality of the adjusted pilot signal(s) meets the qualitytarget.

Three exemplary main embodiments of the method in the NodeB are:

-   -   1) A method in the Node-B which considers the quality of both        the P-DPCCH and the S-DPCCH simultaneously when generating the        ILPC TPC commands.    -   2) A method in the Node-B which considers the quality of either        the P-DPCCH or the S-DPCCH when generating the ILPC TPC        commands. The OLPC may however operate on the packets        transmitted on both the stream associated with the P-DPCCH—the        so-called primary stream—and the stream associated with the        S-DPCCH—the so-called secondary stream.    -   3) A method in the Node-B and the UE where the Node-B only        considers the quality associated with one of the pilots (DPCCHs)        and where the OLPC only is adjusted based on the data        transmission quality observed for the stream that is power        controlled. Transmission power and quality of the stream        associated with the DPCCH which is not power controlled is        controlled by adjusting a power offset used to determine the        number of bits that can be transmitted on the beam that is not        power controlled in a given subframe.

FIGS. 5 a and 5 b are flow diagrams corresponding to the three mainembodiments described above. A first pilot signal and a second pilotsignal are schematically illustrated and denoted with reference numeral51 and 52 respectively. A step 50 of controlling transmission power ofpilot signals using an ILPC is illustrated in FIGS. 5 a and 5 b. In FIG.5 a it is schematically illustrated that the ILPC operates on both thefirst pilot signal (P1) 51 and the second pilot signal (P2) 52, whichcorresponds to the first main embodiment described above. FIG. 5 bschematically illustrates that the ILPC operates on a single pilotsignal only, in the illustrated example on the first pilot signal (P1)51. FIG. 5 b corresponds to both the second and the third mainembodiments described above.

More detailed examples corresponding to the above mentioned three mainembodiments will now be discussed.

In the first main embodiment, the ILPC considers the quality of theP-DPCCH and the S-DPCCH simultaneously when generating TPC commands asmentioned above and as illustrated in FIG. 5 a. Let P_(RX,P-DPCCH) andP_(RX,S-DPCCH) represent the received power associated with the primaryand secondary DPCCH and Γ_(target) represent the desired quality targetused by the ILPC and adjusted by the OLPC. Let furthermoref(P_(RX,P-DPCCH), . . . ) represent a function that maps the receivedDPCCH pilot power, as well as other estimated or known parameters, intoa quality metric. One example of such a mapping would be to compute thesignal to interference ratio, i.e. γ_(P-DPCCH)=P_(RX-P-DPCCH)/I, where Iis the estimated interference level (plus noise) associated with theprimary stream. Then a TPC UP command is generated by the Node-B if

G{f(P _(RX,P-DPCCH), . . . ),f(P _(RX,S-DPCCH), . . .)}<Γ_(target)  (Eq. 5)

and a TPC DOWN command is generated otherwise. Here G is some generalfunction and a typical example would be that the function G correspondsto the min-operator. There are however other functions that could beused to ensure that the quality of both DPCCHs meet the quality targetΓ_(target). Upon receiving a TPC command the UE updates the transmitpower of both the P-DPCCH and the S-DPCCH in accordance with the TPCcommand. This ensures that the relative transmit power differencebetween the P-DPCCH and the S-DPCCH is constant and that the signalquality of both the P-DPCCH and the S-DPCCH meet the quality levelΓ_(target).

The OLPC quality target Γ_(target) is then adjusted by the serving radionetwork controller (S-RNC) based on error statistics (e.g. number oftransmission attempts) associated with the transport blocks transmitted.Here packets on different streams can be treated individually. FIG. 6 aillustrates a corresponding embodiment of a method in the NodeB,comprising the step 50, where the ILPC operates on the first pilotsignal 51 and the second pilot signal 52 as described above withreference to FIG. 5 a. The method illustrated in FIG. 6 a also comprisesa step 60 of the OLPC adjusting the quality target used by the ILPCbased on error statistics. It is schematically illustrated in FIG. 6 athat error statistics of both a first stream (S1) 61 and a second stream(S2) 62 are considered when adjusting the quality target according tothis embodiment. The first stream (S1) 61 is the stream associated withthe first pilot signal (P1) 51 and the second stream (S2) 62 is thestream associated with the second pilot signal (P2) 52.

By using a scheme, according to the first main embodiment, which ensuresthat quality of the worst DPCCH pilot meets the quality target, it willresult in that excessive transmit power is used for the best stream. Forexample, if the SIR associated with the P-DPCCH is x dB higher than theSIR associated with S-DPCCH this means that a fraction 10^(x/10) of thenoise rise budget available to the primary stream is wasted. Hence, thismethod will result in an additional overhead level. An alternative wayto view this is that the best stream relies on an unnecessarily high SIRtarget.

In the second main embodiment the ILPC only considers signal quality ofthe DPCCH pilot associated with one of the two streams. The DPCCH pilotpower associated with the other stream, which is not power controlled,is transmitted at a power such that the relative power differencebetween the two DPCCHs is fixed. Although one in principle could basethe ILPC on either the primary or the secondary stream, the most naturaldesign choice would be to consider the P-DPCCH. Using the latter as anexample a TPC UP command is generated if

f(P _(RX,P-DPCCH), . . . )<Γ_(target)  (Eq. 6)

and a TPC DOWN command is generated otherwise. With this scheme the ILPCwould ensure that the quality level of the primary stream is met. Thequality level of the second stream would however not be considered bythe ILPC.

One example method corresponding to the second main embodiment isillustrated in FIG. 6 b. According to this example the OLPC onlyconsiders the transport blocks transmitted over the power-controlledprimary beam. This is illustrated in FIG. 6 b by the step 50 operatingon the first pilot 51 and the step 60 operating on the first stream 61(which in this example is assumed to be the primary stream). Thus,according to the example illustrated in FIG. 6 b, transmission power ofthe pilot signals is controlled by means of the ILPC operating on thefirst pilot 51 only in the step 50 and the ILPC quality target iscontrolled by the OLPC based on error statistics associated with thefirst stream 61 according to the step 60. Note that some solutions maybe reused by the NodeB to signal to the RNC that it should not considerpackets from the second stream when adjusting the OLPC, cf. SIR targetfreeze. With this approach the quality level for the stream that is nottaken in account by the ILPC will be uncontrollable, in this example thesecond stream. This will result in unpredictable and highly varyingerror statistics for the transport blocks transmitted on this stream.Aside from increasing layer 1 (L1) retransmissions this will alsoincrease the probability for radio link control (RLC) retransmissionsand RLC window stalling.

An alternative example method corresponding to the second mainembodiment is illustrated in FIG. 6 c. According to this example theOLPC is based on error statistics associated with both streams. This isillustrated in FIG. 6 c by the step 50 operating on the first pilot 51and the step 60 operating on both the first stream 61 and the secondstream 62. Thus, according to the example illustrated in FIG. 6 c,transmission power of the pilot signals 51, 52 is controlled by means ofthe ILPC operating on the first pilot 51 only in the step 50 and theILPC quality target is controlled by the OLPC based on error statisticsassociated with both the first stream 61 and the second stream 62according to the step 60. This approach will result in that the OLPCincreases the quality target Γ_(target) so that a sufficient qualitylevel is maintained also for the stream that is not power controlled bythe ILPC. Similarly to the method described above corresponding to thefirst main embodiment, this will result in that an increased overhead isintroduced. It should however be noted that the OLPC is considerablyslower than the ILPC since the OLPC requires communication with the RNC.Therefore, power and quality control of the second stream 62, which isnot power controlled by the ILPC will be considerably slower than thepower and quality control of the first stream 61 controlled by the ILPC.Therefore it may be appropriate to map physical channels which areconsidered to be most important to the stream that is power controlledby the ILPC. Thus a High-Speed Dedicated Physical Control Channel,HS-DPCCH, and/or non-scheduled transmissions from the user equipment maybe mapped to the first stream 61. Given that mapping of the mostimportant channels, HS-DPCCH and/or non-scheduled transmissions, iswell-defined and always mapped to one of the streams, the example methodillustrated in FIG. 6 b could be used to ensure that the quality of thisdata is sufficient.

As mentioned above, in the third main embodiment the ILPC only operateson one of the two streams. In the following we will use the primarystream as an example and thus assume that the ILPC power controls theP-DPCCH. However, it is also possible to instead let the ILPC operate onthe S-DPCCH. Also according to the third main embodiment, the OLPC onlyoperates on the stream that is power controlled by the ILPC whichcorresponds to the method steps illustrated in FIG. 6 b. However, tocontrol the quality level of the data transmissions associated with thestream that is not power controlled the UE dynamically adjust whichE-TFC that should be used, given a certain serving grant, based onfeedback transmitted from the network. More specifically, let SG₂ denotethe grant associated with the secondary stream. This grant can either besignaled explicitly from the network or be derived by the network from agrant SG that needs to be shared between both streams. Then, the totalpower available for data transmissions on the second stream is given as

P ₂ =SG ₂ P _(S-DPCCH)=(SG ₂/δ)P _(P-DPCCH)  (Eq. 7)

where P_(P-DPCCH) is the transmit power of the P-DPCCH, P_(S-DPCCH) isthe transmit power of the S-DPCCH, and δ is the signal power offsetbetween the P-DPCCH and the S-DPCCH. Based on the power available P₂,the UE can compute the corresponding number of bits as if theextrapolation formula is used given that DPCCH quality was met

$\begin{matrix}{K_{e,{ref},m} \cdot \frac{{SG}_{2} \cdot \Delta_{SG}}{L_{e,{ref},m} \cdot A_{{ed},m}^{2} \cdot 10^{\Delta \; {harq}}}} & \left( {{Eq}.\mspace{14mu} 8} \right)\end{matrix}$

where Δ_(SG) represents a power offset that is applied by the UE when itcomputes the number of bits that it can transmit in the given TTI. Notethat 10^(Δharq) is here used to represent the HARQ power profile inlinear scale. In Eq. 8,

$K_{e,{ref},m} \cdot \frac{{SG}_{2}}{L_{e,{ref},m} \cdot A_{{ed},m}^{2} \cdot 10^{\Delta \; {harq}}}$

is the E-DPDCH power extrapolation formula specified in the 3GPPstandard specification TS 25.321 V.9.0.0, section 11.8.1.4, fordetermining a maximum number of bits of scheduled data for an upcomingtransmission using a reference E-TFC (E-TFC_(ref,m)). Note that whenΔ_(SG)=1 equation Eq. 8 simplifies to the legacy procedure for computingthe number of bits that can be transmitted in the TTI. When Δ_(SG)<1,fewer bits will be transmitted than what would be possible using thelegacy E-DPDCH power extrapolation formula (i.e. ignoring Δ_(SG)), whichmay help to keep block error rate (BLER) down if the quality of thesecond stream is poor. Note that it is straightforward to adapt theE-DPDCH power interpolation formula specified in the 3GPP standardspecification TS 25.321 V.9.0.0 in a similar fashion as the E-DPDCHextrapolation formula was adapted above, i.e. the power offset Δ_(SG)can be introduced in the E-DPDCH power interpolation formula forcomputing the number of bits that can be transmitted in a TTI.

The variable Δ_(SG) may, according to some embodiments, be updated bythe NodeB, e.g., in

-   -   a closed loop fashion where the Node-B signals whether the        Δ_(SG) should be increased, reduced, or keep the same value, or    -   an open loop fashion where the Node-B explicitly signals the        value of Δ_(SG) that the UE should employ.

To decide how to update the Δ_(SG) the Node-B could exploit:

-   -   The measured quality of the P-DPCCH and the S-DPCCH. To        exemplify, the difference of the measured signal power        (P_(RX,P-DPCCH) and P_(RX,S-DPCCH)) in dB

P _(RX,P-DPCCH) −δ−P _(RX,S-DPCCH)  (Eq. 9)

-   -    can be viewed as an estimate of how much higher/lower power        offsets that the UE need to use for a certain E-TFC given that        the same amount of power needs to be spent on the E-DPDCH.    -   The error statistics associated with the transmissions of the        stream that is not power controlled by the ILPC. For example, if        number of transmissions required for a certain transport block        exceeds a threshold, the Δ_(SG) could be increased. Similarly if        Δ_(SG) is less than the threshold the power offset Δ_(SG) could        be reduced.    -   Measured received power of the physical channels such as        E-DPCCH, E-DPDCH, DPCCH, etc.

The feedback information conveying Δ_(SG) could be signaled to the UEover a Fractional Dedicated Physical Channel (F-DPCH), an EnhancedDedicated Channel Relative Grant Channel (E-RGCH) or an EnhancedDedicated Channel Absolute Grant Channel (E-AGCH).

Note also that the power offset Δ_(SG) can be applied to either all or asubset of the physical HSUPA channels (E-DPCCH, E-DPDCH, and/orHS-DPCCH) transmitted on the stream that is not power controlled by theILPC.

According to some alternative embodiments, the variable Δ_(SG) may beupdated by the UE based on error statistics associated with the streamthat is not power controlled by the ILPC. Such error statistics may beobserved by the UE. The error statistics may e.g. be based on feedbackinformation transmitted on a E-DCH Hybrid ARQ Indicator Channel (E-HICH)associated with the stream that is not power controlled by the ILPC.

FIG. 7 is a flow diagram illustrating an example method in a NodeBcorresponding to the above described third main embodiment. The methodcomprises a step 50 of controlling transmission power of a UE configurefor uplink MIMO by using an ILPC operating on a first pilot signal. Themethod further comprises a step 60 of adjusting a quality target used bythe ILPC based on error statistics associated with a first streamassociated with the first pilot signal, i.e. error statistics of anyother stream which is not associated with the first pilot signal is nottaken into account when adjusting the quality target of the ILPC. In astep 71, the NodeB determines how to update a power offset for computingnumber of bits that can be transmitted, in a given TTI, on physicalchannel(s) in a second stream which is not power controlled by the ILPC.In a step 72, the UE signals to the UE to update the power offset to beapplied by the UE for computing the number of bits to transmit in a TTI.

From the above description of exemplary methods in the NodeB, it can beunderstood that corresponding methods may be implemented in the UE. Someof these corresponding methods require a modified behavior of the UEcompared to the behavior the UE would have in a scenario of singlestream transmissions. FIG. 8 is a flow diagram illustrating a method ina UE configured for uplink MIMO. The method comprises a step 81 oftransmitting multiple pilot signals and a step 82 of receiving TPCcommands for adjusting the transmission power of a first pilot signal.The method further comprises a step 83 of adjusting the transmissionpower of the first pilot signal in accordance with the transmit powercontrol commands while adjusting the transmission power of a secondpilot signal so that a fixed power difference is maintained between thefirst pilot signal and the second pilot signal.

FIG. 9 is a flow diagram illustrating an exemplary method in a UE whichcorresponds to an embodiment according to the third main embodimentdescribed above. The method illustrated in FIG. 9 comprises steps 81-83corresponding to steps 81-83 described in connection with FIG. 8. Themethod also comprises an optional step 91 of receiving, from the NodeB,signaling for updating a power offset to be applied by the userequipment when computing the number of bits that can be transmitted in aTTI on a physical channel transmitted on the second stream associatedwith the second pilot signal. In a step 92 the power offset is updatedin accordance with the signaling received in the step 91. Alternativelythe UE may autonomously determine how to update the power offset for thebit computation in the step 92 based on error statistics associated withthe second stream. Thus the step 91 is not required in cases where theUE autonomously determines how to update the power offset. The method ofFIG. 9 also comprises an additional step 93 of computing the number ofbits the user equipment can transmit on the second stream based on theupdated power offset.

FIG. 10 is a schematic block diagram of an exemplary embodiment of abase station 12, such as a NodeB, which may be configured to carry outthe example methods illustrated in FIGS. 5-7. As illustrated in FIG. 10,the base station 12 includes a processor 101, a memory 103, atransceiver 102, a network interface 104 and an antenna 108. The antenna108 may comprise multiple antenna elements configured for uplink and/ordownlink MIMO. In particular embodiments, some or all of thefunctionality described above as being provided by a NodeB, may beprovided by the base station processor 101 executing instructions storedon a computer-readable medium, such as the memory 103 shown in FIG. 10.Thus the processor 101 may be configured to execute instructions ofdifferent software modules, such as a software module 105 comprisingprogram instructions for implementing the ILPC of the differentembodiments described above, or a software module 106 comprising programinstructions for interacting with a RNC, which implements the OLPC, toadjust the quality target of the ILPC according to the differentembodiments described above. Alternative embodiments of the base stationmay include additional components responsible for providing additionalfunctionality, including any of the functionality identified aboveand/or any functionality necessary to support the embodiments describedabove.

FIG. 11 is a schematic block diagram of an exemplary embodiment of a UE13, which may be configured to carry out the example methods illustratedin FIGS. 8 and 9. As shown in FIG. 11, the example UE 13 includes aprocessor 111, a memory 113, a transceiver 112, and antennas 23 and 24.The antennas 23 and 24 may be embodied as different antenna elements ofa multi-element antenna. In particular embodiments, some or all of thefunctionality described above as being provided by a UE, may be providedby the UE processor 111 executing instructions stored on acomputer-readable medium, such as the memory 113 shown in FIG. 11. Thusthe processor 111 may be configured to execute instructions of differentsoftware modules, such as a software module 114 comprising programinstructions for implementing UE functionality with respect to the powercontrol based on received TPC commands according to the differentembodiments described above, or a software module 115 comprising programinstructions for implementing the power offset adjustment with respectto bit computation of the different embodiments described above.Alternative embodiments of the UE may include additional componentsbeyond those shown in FIG. 11 that may be responsible for providingcertain aspects of the UE's functionality, including any of thefunctionality described above and/or any functionality necessary tosupport the embodiments described above.

From the description above it is apparent that some of the embodimentsof this disclosure enables improved network control of quality of thepacket transmissions for uplink MIMO transmissions.

The embodiments of this disclosure are applicable to both single-celland dual-cell WCDMA/HSUPA systems. Furthermore, although the embodimentsare described in a context of a UE and a Node-B equipped with twotransmit antennas/antenna elements and for the UE architecturesdescribed in FIGS. 2, 3 and 4, the disclosure is also applicable tosettings with a larger number of transmit and receive antennas.

In the drawings and specification, there have been disclosed typicalembodiments and, although specific terms are employed, they are used ina generic and descriptive sense only and not for purposes of limitation,the scope of the invention being set forth in the following claims.

What is claimed is:
 1. A method in a user equipment configured foruplink Multiple-Input Multiple-Output (MIMO) transmissions, the methodcomprising: transmitting multiple pilot signals, including a first pilotsignal associated with a first MIMO stream and a second pilot signalassociated with a second MIMO stream; receiving, from a NodeB, transmitpower control commands for adjusting the transmission power of the firstpilot signal; adjusting the transmission power of said first pilotsignal in accordance with said transmit power control commands;receiving, from the NodeB, signaling for updating a power offset to beapplied by the user equipment when computing the number of bits that canbe transmitted in a time transmission interval on at least one physicalchannel transmitted on a second MIMO stream associated with a secondpilot signal; updating said power offset in accordance with saidreceived signaling; and computing the number of bits the user equipmentcan transmit on said second MIMO stream based on said power offset. 2.The method of claim 1, wherein said received signaling comprises a valueof said power offset.
 3. The method of claim 1, wherein said receivedsignaling comprises information indicating whether a value of said poweroffset is to be increased, reduced or maintained.
 4. The method of claim1, further comprising: updating a power offset to be applied by the userequipment when computing the number of bits that can be transmitted in atime transmission interval on at least one physical channel transmittedon the second MIMO stream associated with said second pilot signal,wherein said power offset is updated based on error statisticsassociated with said second MIMO stream; and computing the number ofbits the user equipment can transmit on said second MIMO stream based onsaid power offset.
 5. The method of claim 4, wherein said errorstatistics associated with said second MIMO stream are based on feedbackinformation transmitted on a E-DCH Hybrid ARQ Indicator Channel (E-HICH)associated with said second MIMO stream.
 6. A user equipment configuredfor uplink Multiple-Input Multiple-Output (MIMO) transmissions, the userequipment comprising a transceiver and a processor, wherein thetransceiver is configured to: transmit multiple pilot signals, includinga first pilot signal associated with a first MIMO stream and a secondpilot signal associated with a second MIMO stream; and receive, from aNodeB, transmit power control commands for adjusting the transmissionpower of the first pilot signal; receive, from the NodeB, signaling forupdating a power offset to be applied by the user equipment whencomputing the number of bits that can be transmitted in a timetransmission interval on at least one physical channel transmitted on asecond MIMO stream associated with a second pilot signal; and whereinthe processor is configured to: adjust the transmission power of saidfirst pilot signal in accordance with said transmit power controlcommands; update said power offset in accordance with said receivedsignaling; and compute the number of bits the user equipment cantransmit on said second MIMO stream based on said power offset.
 7. Theuser equipment of claim 6, wherein the processor is further configuredto: update a power offset to be applied by the user equipment whencomputing the number of bits that can be transmitted in a timetransmission interval on at least one physical channel transmitted onthe second MIMO stream associated with said second pilot signal, whereinthe processor is configured to update said power offset based on errorstatistics associated with said second stream; and compute the number ofbits the user equipment can transmit on said second MIMO stream based onsaid power offset.
 8. The method of claim 7, wherein said errorstatistics associated with said second MIMO stream are based on feedbackinformation transmitted on a E-DCH Hybrid ARQ Indicator Channel (E-HICH)associated with said second MIMO stream.